Re: [PATCH v1 2/3] mm: memcg: read root lruvec LRU stats from the node counters

[sashiko-bot@kernel.org]

> Reclaim and refault detection read LRU sizes and workingset counters
> from the target lruvec after a ratelimited rstat flush. For global
> reclaim and the root memcg, that flush covers every cgroup in the
> system, and the ratelimiting can leave the stats stale for seconds.
> 
> For stats that only track memory charged to a memcg (LRU sizes,
> workingset, rotation and reclaim writeback counts), the root memcg's
> hierarchical value equals the node counter, which is always up to date.
> Add lruvec_lru_page_state() and a monotonic variant that read the node
> counters for the root memcg, and use them in reclaim and
> workingset_test_recent(). Root reclaim and refault detection then see
> current values instead of stats that can lag by seconds.
> 
> workingset_test_recent() only reads the eviction memcg's own lruvec, so
> skip its rstat flush when that is the root memcg. For reclaim however,
> we still need to keep the flush (for now) since global and root reclaim
> also read the rstat stats of the child memcgs they scan, which the flush
> is needed for in order to keep up to date.
> 
> On an 80-CPU machine, a root flush took 69us on average with little
> cgroup activity and ~0.9ms with 128 memcgs actively allocating (wall
> time, including waiting for the rstat lock). Reading a node counter
> doesn't add any cost. It took the same amount of time as reading the
> rstat counter (~150ns, which includes bpftrace probe overhead).
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261102225030.2743129-1-joannelkoong@gmail.com?part=2